Addie B. and her cousins Jack, Beanie, and Bodie are mourning the loss of their grandmother Winnie. While visiting their grandfather’s beach house in Maine, Addie receives a mysterious treasure box once owned by her grandmother. In it is a moonstone ring and a mysterious poem from her grandmother. The poem leads Addie and her cousins to nearby Moonstone Cave where, together, they enter a secret and magical garden and embark on an adventure of a lifetime. Along the way, they meet wonderful friends like Gemma, who reminds them all of a younger version of their grandmother; Jumani, a large white wolf; and Jadira, a strange and smelly creature whom they grow to love. During the adventure, they learn the most important lesson of all—they must work together if they want to escape the dangers ahead of them and find their way back home.